In OS/2 2.1, Seamless Windows is supported by allowing the Windows display driver to draw directly on the Presentation Manager (PM) screen. This means that Seamless Windows updates do not go through the PM drawing functions and, therefore, will not update the active screen change area (SCA) in the usual way. As a result, Seamless Windows requires special treatment.

Prior to drawing on the PM screen, the Seamless Windows driver calls the PM driver through an exported entry point, SeamlessExcludeCursor. This call excludes the PM cursor from the area in which the Seamless Driver is about to draw. The modified DCAF-enabled display driver intercepts this call and passes the rectangle coordinates to AccumulateScreenBounds.

Under DCAF, during PM display driver initialization, Seamless Windows must be granted addressability to all data and code that it will access during the call to SeamlessExcludeCursor. In order to add the supplied rectangle to all active SCAs, which can reside anywhere in the display driver heap, there is a single, static SSA (Seamless Screen Area), called scaSeamless, used for this purpose. All Seamless bounding rectangles will be accumulated in scaSeamless; then, this SCA is merged with the other active SCAs when a query is issued via GreGetScreenChangeArea.

As a result, at initialization (in InitializeSeamless), the Seamless Windows driver is given access to AccumulateScreenBounds, to scaSeamless and to the DDT display driver control block, in order to retrieve the screen dimension at seamless time. The addresses of this data are stored in the SeamlessAddresses control block, owned by the Windows driver.

Before writing to the PM screen, the Seamless Windows driver will call SeamlessExcludeCursor. The exclusion rectangle will be passed in the following registers in so-called AI coordinates (i.e., 16-bit inclusive; 0,0 is top left of screen):

Left

Top Right Bottom

The display driver will determine whether the new bounds accumulation is needed by checking the value of the pStartSCA pointer and, in SeamlessExcludeCursor32, will call AccumulateSeamlessBounds to initiate the bounds accumulation. AccumulateSeamlessBounds converts the passed rectangle to EXCLUSIVE SCREEN coordinates, clips the rectangle to the screen dimensions and calls AccumulateScreenBounds. This causes the rectangle supplied to SeamlessExcludeCursor to be added to only scaSeamless. When an SCA is queried using GreGetScreenChangeArea, the Seamless SCA is merged with the active SCAs and then reset to NULL.

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